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Properly Sizing a Datto Appliance



How do I determine the correct Datto appliance size for my client's needs?


  • Datto SIRIS
  • Datto ALTO
  • Datto NAS


This article will help you determine the best-sized Datto business continuity appliance for your data backup needs. We recommend that you review the information presented here before you upgrade or purchase a Datto appliance.


Considerations for appliance sizing

When qualifying clients for a Datto appliance, it is important to consider both the current and future storage requirements.

For Datto NAS, calculate the average amount of storage space your production shares require, and use a 2x multiplier to determine your most optimal storage scenario.
  • How many machines will you need to protect?
  • Are these machines workstations or servers?
  • What is each machine's total protected size? To determine this amount, calculate the currently utilized space on the appliance, and then add a small cushion to account for future growth. You will need about 30% free space on the backed up hard drive to allow for optimal backups. The bare minimum is 20% (Though anything under 30% has the potential to throw errors during the backup process).
  • How much block level change will the system produce? Take into account databases, separate database backups, log files, application, and files change on a daily basis.
  • Are you going to have encrypted agents? Encrypted agents should be treated as uncompressed and will take up more space.
We recommend that you underestimate this number, as block level changes take into account many more factors than just file changes: it also involves disk sector changes.
  • How often should you back up this system? Typically, Exchange servers require an hourly backup, terminal servers require a daily backup, and auxiliary domain controllers only require a few backups a week.
  • How far back would a client desire to go back for local, expedient restoration of files, folders, or entire operating systems?
  • If virtualized, how long would you need to run this virtual machine off of the Datto appliance?
  • Does the client have expansion plans in place to possibly include additional servers or workstations?
  • What type of files are being backed up? Certain file types cause large changes when accessed for utilization or updating. Large databases, media files, or production workspaces can have large sizes and thus larger backup images.
For Datto Windows Agent volume space requirements, Refer to the System Requirements section of this article.

Datto best-practices

The base recommendation for a Datto appliance is two times the total protected space of the agents that are to be protected. This is the 2-3x multiplier rule, which ensures that the appliance will have the storage space needed to ensure the following:

  • The Datto appliance has storage based for the first backup, and then continuously roll this base image to the front of the chain with each backup. Backups will start to fail as the appliance reaches near full capacity.
  • The Datto appliance has storage space for backups based on the Datto recommended retention policy, as shown in Figure 1.

Figure 1: Retention policy

  • The Datto appliance has storage space for local virtualization of agents. When an agent is virtualized on appliance, the Datto solution continues to backup the virtualized system that is on the network. If the local virtualization had the Rescue Agent option enabled, then subsequent backups will be incremental. If not, then the next backup will be a new full base image. The immediate backup after a completed disaster recovery scenario (i.e. a completed bare metal restore, image export, etc.) may result in a new full base image as well.
  • There is space for the appliance's cloud synchronization. If there is not enough available space to create the offsite transfer file, the appliance could potentially have stalled cloud synchronization.
  • There is a slight safety buffer in storage size for cases in which mild unexpected data growth could potentially occur over time. When a Datto appliance's primary storage space reaches maximum capacity the optional resolution paths may be more difficult.

Encrypted Agents

If you are planning to use encryption to protect your agents, note that these backups are stored uncompressed. It may be advisable to look for a 3X multiplier when working with encrypted agents.

Datto hardware resource limitations

Datto appliances must have adequate resources to perform daily tasks such as backups, and ad hoc, resource intensive operations such as a local virtualization. Low system resources can severely affect local virtualizations, as the virtualized machine will be seeking resources that are not physically available. This can lead to virtual machines running extremely slow and even high-resource programs crashing.

For example:

  • A client has a single server environment (Small Business Server) and the server runs the daily operations of the entire organization.
  • This server has a quad-core CPU and 32GB of RAM, storing on average 525GB of data on its HDD.
  • If sizing the appliance on storage alone, the first reaction would be to purchase an S3B1000 appliance to accommodate the space of the server. An S3B500 fits within the 2-3x multiplier perfectly, and even provides ample space for data growth.
  • However, should the server go down and clients need to run off of the server, the resources provided on the machine may not be able to power the machine in a sufficient state in order to allow for operations to continue.
  • An S3B1000 has 32 GB of RAM and 4 CPU cores total. The best possible scenario for the server to run would be running 24GB of RAM, and taxing all 4 cores. Over-allocating RAM will hinder basic operations on the Datto appliance, while an under-provisioned virtual machine will not function as intended during the virtualization.

To ensure all of the appliance's features function properly, the client will want to consider a larger appliance, such as an S3B2000, that has more resources in order to allow for more resources to be considered for virtualization.

Datto Appliance Sizing FAQs

What happens if I don't have a large enough appliance?

The following problems result from not having enough space on your Datto appliance:

  • It fills quickly after deployment, blocking future backups.
  • Inability to maintain local and off-site backup chains.
While Datto Technical Support may be able to provide a temporary remedy, issues stemming from the lack of free space will return until space is freed on the appliance, or it is upgraded.

Assessing the size of the appliance

To check the size of the appliance, access the Device Overview page.

Figure 2: Device overview (click for larger image)

Use the Local Storage Usage graphic and list to assess the size of all protected machines on your Datto appliance, and the remaining local free space.

When is it time to upgrade?

If you are experiencing failed local backups and are constantly modifying local backup retention in order to keep your system protected, it may be time to upgrade your appliance to the next size up.

As a rule of thumb, if your appliance is utilizing 85% of more of its local storage from backup storage, or if you do not have enough space to virtualize your largest agent, it may be time to discuss upgrading options.

A "real world" sizing scenario

A small client network has four machines that need to be protected:

  • Exchange Server: 250 GB
  • Application Server: 45 GB
  • Terminal Server: 60 GB
  • Workstation: 100 GB

The total protected space here comes out to 455 GB. This is on the border of an S3B1000 appliance with a terabyte of storage. For local backups, this should be a suitable purchase. There are considerations to take into account:

How long do they want to maintain local data?

  • Do they have enough upstream bandwidth to get the images synced with the cloud on a consistent basis?
  • The recommendation is to have a minimum of 100Kb/s upstream on a consistent, uninterrupted basis per terabyte of local storage. More is always advisable.

What if they had to restore the Exchange Server? How would this impact space?

  • If the Exchange server goes down and needs to be brought up locally on the Datto appliance via an instant virtualization, once that machine has been established on the network, the next backup will be a base image as the hardware platform has changed.
  • Factor in the next Exchange backup being 250 GB. That will cause your appliance to quickly fill up with an additional base image being present, not counting incremental changes subsequently
  • This may not be as pressing of a factor for smaller agents as there is less to backup should the machine need to be virtualized.

What kind of performance are they expecting to get out of these machines?

  • Depending on the resources of the protected machines, should they need to be virtualized, what kinds of processing power and RAM will be required for the machines to operate?
  • What is the daily activity level on each of the machines? How many people are accessing the machines on a regular basis?
  • Could the client survive with a virtual machine in an under-powered state that is being used in production?

If your specific sizing scenario is not covered here, contact your Datto Sales Executive for assistance.

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